The present invention starts out from a method for shifting the instant of commutation for a sensorless and brushless direct-current motor whose stator windings are fed by a multi-phase converter connection according to the definition of the species in claim 1 and also relates to a system for implementing this method according to claim 6.
DE 39 40 568.9 A1 describes a circuit configuration for operating a multi-phase synchronous motor at a direct-current supply. In this context, the phases are successively connected to the direct voltage and commutating circuits corresponding to the rotor position are controlled in such a manner that they overlap with respect to time for commutating subsequent phases, and at least one of the commutating circuits in the commutation range is clocked in such a manner that the average value of the current increases in the forward commutating phase and decreases in the reverse commutating phase. As a result of this overlapping and clocking of the switching signals in the commutation edges, there is less switching loss and a reduction in noise.
For sensorless and brushless direct-current motors, the instant of commutation is typically determined by measuring the induced voltage in a particular non-current carrying stator winding phase. In this context, this induced voltage is compared to a reference voltage that is derived from the actual value of the rotational speed. In this connection, significant power notches and ripples in the torque can occur, particularly in the case of large loads and high motor speeds. This is extremely disadvantageous.
The object of the present invention is to provide a method that enables the instant of commutation to be shifted for a sensorless and brushless direct-current motor so as to prevent or significantly reduce the power notch, and decrease the torque ripple.
With respect to the related art, the method according to the present invention for shifting the instant of commutation for a sensorless and brushless direct-current motor having the characteristic features of claim I has the advantage of an increase in power with a constant magnetic circuit and identical motor mechanics, and of a reduction in the torque ripple by adapting the commutation threshold to an optimum current waveform. Advantageously, there is also no power notch observed, as is in the case of a commutation shift dependent on the measured motor speed.
In the method according to the present invention, this is principally achieved in that commutation is detected by comparing the voltage induced in a non-energized stator winding phase to a reference voltage, and in that the reference voltage is changed in dependence upon the setpoint value of the motor speed and/or the manipulated variable calculated therefrom.
Advantageous further refinements and improvements of the method stated in claim 1 are rendered possible by the measures specified in the additional method claims.
According to a particularly advantageous and preferred specific embodiment of the method according to the present invention, the instant of commutation is shifted ahead with respect to time in such a manner that an optimum current waveform is achieved, i.e., optimum particularly with regard to an increase in power and/or a reduction in the torque ripple.
According to a particularly effective and advantageous embodiment and further refinement of the method according to the present invention, the instant of commutation is shifted in such a manner that the reference voltage is raised in the shape of a parabola.
In a further advantageous embodiment of this method feature, given a pulse width modulation of the current supplied to the stator windings, the reference voltage is raised in the shape of a parabola, beginning at a pulse width modulation ratio of about 90 to 95%, in particular 93%. Raising the commutation threshold in the shape of a parabola has the advantage that it results in a smoother transition to the pre-commutation state.
According to a further advantageous feature of an exemplary embodiment of the method according to the present invention, besides being used for changing the reference value for the instant of commutation, the manipulated variable determined in dependence upon the setpoint value of the rotational speed is also used for adapting the current supply to the individual stator winding phases, raising it or lowering it accordingly.
A preferred system for implementing the above-explained method with its different modifications includes a sensorless and brushless direct-current motor that is fed by a multi-stage converter connection, which, for its part, includes an output stage control, a commutation logic, a phase selector, and a phase discriminator, and is characterized in that a commutation detection is provided which is supplied at one input by the phase selector with the instantaneous value of the voltage induced in a non-energized phase and, at a second input, with a reference voltage, for comparison, and in that the reference voltage can be changed by a commutation shift in accordance with a specific curve, a manipulated variable being supplied to the commutation shift by a manipulated-variable calculation as a function of the setpoint speed of the motor.
In an advantageous embodiment of this system according to the present invention, it is provided that in the commutation shift, the reference voltage changes in accordance with a parabola, in particular, it is increased.
Given a pulse width modulation of the current supply to the individual stator winding phases of the motor, one advantageous embodiment of this system configuration provides that the reference voltage is increased in the shape of a parabola, starting from a pulse width modulation ratio of about 90 to 95%, preferably 93%. These percent values apply for a specific magnetic circuit design. Other designs of the magnetic circuit can result in significantly different values.
In a further advantageous and particularly effective configuration of the system according to the present invention, calculating the manipulated variable yields, as a non-linear function of the setpoint speed of the motor, a manipulated variable that, on the one hand, is supplied to the commutation shift as an input, and, one the other hand, is supplied to the commutation logic for adapting the current supply to the stator winding phases of the motor.